(1) Field of the Invention
This invention relates to alternating type phase shifting photomasks and more particularly to multi-phase multi-layer phase shifting photomasks using phase shifting layers of less than 180.degree. optical phase shift to form a mask having a total optical phase shift of 180.degree..
(2) Description of the Related Art
The use of phase shifting technology in masks used in projection systems to expose a layer of photoresist formed on a semiconductor substrate has become very important as the requirements of image definition and depth of focus have become more stringent. A number of workers have disclosed methods of forming and using phase shifting photomasks.
U.S. Pat. No. 5,358,808 to Nitayama et al. and U.S. Pat. No. 5,376,483 to Rolfson teach etching into a transparent mask substrate in addition to the formation of phase shifting material to provide photomasks having phase shifting regions with different optical lengths and different amounts of phase shift.
U.S. Pat. No. 5,403,682 to Lin teaches the use of phase shifting rims surrounding each pattern of a photomask.
U.S. Pat. No. 5,429,897 to Yoshioka et al. teaches the use of attenuating type phase shifting masks.
U.S. Pat. No. 5,393,623 to Kamon teaches the use of reflection type phase shifting photomasks.
U.S. Pat. No. 5,387,485 to Sukegawa et al. teaches the use of photomasks using annular arrangement of light shielding regions together with phase shifting material.
U.S. Pat. No. 5,246,800 to Muray teaches an alternating type phase shifting mask using a transition region having three different optical phases. The three phases are assigned to pixels, or subregions, in the transition region.
In the invention of this Patent Application uses an alternating type phase shifting photomask. The phase shifting region of the photomask is formed from a number of layers of phase shifting material. The total optical phase shift of the mask is 180.degree. and the phase shift of each layer is less than 180.degree.. The use of a multi-layer phase shifting photomask provides greater control of the phase shift during mask fabrication and a gradual transition between no phase shift and a phase shift of 180.degree. which improves the quality of the image formed by the photomask.
The multi-layer mask is formed with an etching process having a wide tolerance with regard to etching times. The thickness control for the phase shifting layers is provided by control of the deposition of the layers.